Seven people go on trial on Monday for the multi-million pound theft of a Monet, Sisley and two Breughels in Nice but their leader claims the FBI's art crimes chief "ordered" the heist.

The men face maximum sentences of 30 years in prison for armed robbery at the end of the week-long trial in Aix-en-Provence. The leader's lawyer claims they were a bunch of bumbling art amateurs talked into the heist by the world's most notorious art detective bent on catching bigger prey.

At lunchtime on August 5, 2007, thieves dressed in blue overalls and ski masks burst into the poorly guarded Musée des Beaux Arts.

Their leader, Pierre Noël-Dumarais, then 60, pointed a Colt 45 at the welcome desk while four accomplices unhooked four paintings from the museum walls and stuffed them into black bin bags. Five minutes later, they made their escape in a blue Peugeot van.

In the boot were two Breughels – Allegory of Water and Allegory of Earth – Alfred Sisley's Avenue of Poplars at Moret and Claude Monet's Cliffs Near Dieppe. While their combined value has been estimated at 22 million euros, their stolen sale price would be no more than three million euros.

The French police had few leads bar DNA from a cigarette butt and a bin bag, but they would soon receive help from across the Atlantic.

Robert K Wittman, then FBI special agent and chief of its Art Crimes Team, first got wind of the paintings while undercover as a shady American dealer moving stolen art for crime syndicates and drug lords. He was told about the works by Miami-based Frenchman Bernard Jean Ternus, with links to Marseille's Brise de Mer Corsican mafia clan.

Now retired, "Bob" Wittman recovered around $300 million-worth of stolen art and objects in his 20-year career, including Geronimo's war bonnet, one of the original 14 copies of the US Bill of Rights, and works by Rembrandt, Rodin and Rockwell.

But the greatest unsolved art crime in history still eluded him, namely the 1990 theft from the Isabella Steward Gardner Museum in Boston of a long-lost Vermeer, two Rembrandts and five sketches by Degas worth around $500 million.

His pulse raced when his plump, shaggy-haired French connection, Mr Ternus, alias "Sunny" boasted that he could get hold of the Vermeer and a Rembrandt.

To "hook" Sunny, Mr Wittman invited him to a party on a Miami yacht, complete with bikini-clad models and staged the sale of five fake masterpieces to a Colombian drugs baron in exchange for gold and diamonds. The entire crew and cast were FBI agents, but Sunny fell for it.

This is where the version of events diverges.

Mr Wittman says that during subsequent conversations about the Rembrandt and Vermeer, Sunny offered him and his co-agents other works, including two Picassos and the Nice paintings.

"I had no idea about the Nice theft nor had I ever met or spoken to the defendants until after it occurred," Mr Wittman told The Daily Telegraph.

"We couldn't turn down (the sale offer) as they were all crimes."

However, Ludovic Depatureaux, the lawyer of the gang's alleged leader, Mr Noël-Dumarais, claims that Mr Wittman encouraged the theft by expressing an interested in "Dutch paintings".

"In autumn 2006, he effectively placed an order with Bernard Ternus, saying he was interested perhaps in Vermeer and Rembrandt, but in Dutch paintings in general, and had buyers.

"Wittman thought that (via the Nice thefts), he would infiltrate those who stole or still hold the Vermeer and Rembrandt. The Nice heist was just collateral damage.

"My client's modus operandi did not start from the premise: let's steal some paintings then find a buyer. They were a bunch of amateurish 'stooges' some of whom only met on the day of the heist.

"These canvasses disappeared in order to recover two key paintings belonging to US heritage. I'm not sure that the US would appreciate it if French agents acted likewise."

He said Mr Wittman was guilty of "police provocation" and that he would call for charges to be dropped.

Mr Wittman said: "In the US, we law enforcement officers used to call that 'throwing fecal matter against the wall and seeing what would stick'".

"I don't think anything I did 'encouraged' anyone to obtain Chechen hand grenades and semi automatic pistols in order to commit armed robbery. It is a fanciful defence at best, at worst, it is a defence of desperation used only when criminals are caught."

He denied they were amateurish. "They were good criminals, but terrible businessmen."